Rankine cycles utilizing water and steam and/or low boiling point organic fluids have been proposed to recover heat energy from exhaust. These systems have not been efficient enough to justify commercialization. Regenerative braking systems which retard a vehicle and store the braking energy for restart are known. They are heavy and costly. Their use is justified only in city buses which stop and start frequently.
Currently, "adiabatic" or low heat rejection engines (LHRE) are being developed in which ceramic materials are used to insulate the internal parts adjacent to the combustion area. The actual gain in fuel efficiency is only 3 or 4%. The objective appears to be to make the engine lighter, simpler and cheaper by reducing the size and complexity of the cooling system or eliminating it altogether. In the exceptionally efficient conventional diesel engine, the conversion of fuel energy is generally as follows: 38-40% is converted to mechanical power, 25% is rejected to the engine's cooling and lubricating fluids, and 35% as thermal energy in the exhaust. The LHRE design with its reduction of heat loss to the cooling system could result in an exhaust heat content of 42%, and the percentage would increase the nearer the design approached the non-cooled state, so that if cooling were eliminated,, the exhaust energy content could approach 55-57%.
Further developments are taking place to utilize this heat lost to the exhaust. An experimental Rankine cycle with a once-through boiler but no drum and using a synthetic working fluid has demonstrated an improvement of fuel economy of 12% in a conventionally cooled diesel powered truck. Studies show that with the LHRE attaining an exhaust temperature of 1,240-1600.degree. F., an efficiency of 15-35% could be attained using water and steam. Previous Rankine cycles utilized a once-through boiler which had no storage drum with a feed pump pumping the fluid through coils in the exhaust passage.
Various retarders have been developed to aid heavy duty vehicles in slowing down during a braking period, thus reducing the need to apply the friction brakes and saving brake maintenance costs and downtime. The retarders employed include: (1) an exhaust brake, in which an auxiliary valve in the engine exhaust system cuts off the exhaust gas flow through the exhaust pipe and creates a high back pressure, thus increasing the pumping loss of the engine; (2) an electromagnetic retarder in which a resistance motor is connected with the drive shaft of the vehicle and can be activated to produce braking resistance; and (3) a hydrodynamic retarder which is similar to a fluid clutch in which an oil submerged turbine rotor revolves against a stator. These retarders add to the cost, weight and maintenance of the vehicle.
There have also been developed and offered on the market what are called regenerative braking systems. These systems offer the advantages of the retarders, are more effective, and conserve the braking energy or the vehicle for use when the vehicle accelerates from a stop.
Gyroscopic regenerative systems use braking energy to speed up a gyro wheel through a constantly variable transmission (CVT). The braking energy is stored in the rapidly revolving wheel, and the wheel is connected through the CVT to accelerate the vehicle from a stop. In hydrostatic regenerative braking systems, the energy of braking is utilized to pump oil from a storage receptacle or tank to a high pressure in a gas (i.e. nitrogen) filled tank (to a pressure of over 5,000 or 6,000 psia). On getting under way, the pressurized gas forces the oil through an oil motor geared to the transmission or to the drive shaft of the vehicle to start the vehicle moving or to assist the vehicle in accelerating. These systems add to the cost and weight of the vehicle. They save on fuel cost and brake overhaul and have environmental benefits, but the investment is justified only in city buses which stop with great frequency.